1. Field of the Invention
The present invention relates to an aperture and a projection lens system, and particularly to an aperture having a plurality of filter portions and a projection lens system having the aperture.
2. Description of Related Art
FIG. 1A is a diagram for illustrating a structure of a conventional projection lens system 100, and FIG. 1B is a diagram for illustrating a structure of an aperture 120 shown in FIG. 1A. Referring to FIGS. 1A and 1B, the conventional projection lens system 100 comprises a plurality of lenses 110 and the aperture 120, the aperture 120 being disposed among the lenses 110. The projection lens system 100 is adapted to project beams 80 onto a screen (not shown) for forming images. The aperture 120 has a transmission portion 122 and an absorption portion 124, in which the transmission portion 122 is adapted for allowing the beams 80 to pass through and the absorption portion 124 is adapted for absorbing the beams 80. In addition, a diameter of the transmission portion 122 is relative to a value of f-number, which is the focal length divided by the diameter of the transmission portion 122, and usually is represented by f/number. The larger the number is, the smaller the diameter of the transmission portion 122 is. And the smaller the number is, the larger the diameter of the transmission portion 122 is.
FIGS. 2A to 2C are spot diagrams of a blue light, green light and red light when an aperture with an f-number of f/2.1 is used in a projection lens system. Generally, after passing the apertures having the same f-number, a beam having shorter wavelength forms a spot diagram having a larger diameter. Therefore, referring to FIGS. 2A to 2C, the diameter of the spot of the blue light is the largest and the diameter of the spot of the red light is the smallest. However, the diameters of the spots affect the image quality, and the diameter of the spot of the blue light shown in FIG. 2A is too large to produce desired image quality.
FIGS. 3A to 3C are spot diagrams of a blue light, green light and red light when an aperture with an f-number of f/2.4 is used in a projection lens system. Referring to FIGS. 3A to 3C, generally, for improving the image quality, an aperture having a larger number (or having a relatively small diameter in the transmission portion 122) is used to decrease the spot. Compared with those shown in FIGS. 2A to 2C, all the spots of blue light, green light and red light in FIGS. 3A to 3C are smaller. Therefore, an aperture having a larger number of the f-number can improve the image quality.
However, by using the aperture having the larger number of the f-number, the diameter of the transmission portion 122 (shown as FIG. 1) is smaller, and the light influx is less, thus a brightness of the image is also reduced.
In view of the above, when adopting the aperture having the smaller number, the conventional lens system produces images having higher brightness but worse image quality, when adopting the aperture having the larger number, the conventional lens system produces images having lower brightness but better image quality. Therefore, the conventional projection lens system cannot produce images having better image quality and higher brightness of the images at the same time.